Almost all large wind turbines build today have pitch bearings at the blade root so that the pitch of the whole blade can easily be changed by activating the pitch mechanism. Pitching of the blade is used as a brake to stop the rotor but it is also used for power and load control in different ways. On active stall controlled turbines a relative slow variation of the pitch of the blades is used to adjust the pitch so that stall on the blade occurs at the right maximum power, but the pitch system is also active at low wind speed to maximise the power. On pitch controlled wind turbines the pitch at high wind is changed continuously, so that the maximum power is not exceeded. This is done by reducing the inflow angle to the blade when the wind speed is increasing. The pitch system is also used at low wind speeds to maximise the power.
Recently, new types of pitch regulated wind turbines also use the pitch system to reduce the dynamic loads, either by cyclic pitch or by individual blade pitch. Different input signals to the control system as e.g. the flapwise loading can be used.
Apart from this state-of-the-art control with blade pitching, control of power and loads by movable, small control surfaces have been proposed [2]. A 550 kW turbine designed by ZOND in USA used a partial span aileron system for control of power and rotor rotational speed, [1]. The aileron system is similar to trailing-edge flaps for aeroplanes. Gurney flaps attached to the trailing edge of the blades have as well been proposed and analysed [3] (cf. FIG. 1 of [3]). The advantage of a small control surface is a possible faster response due to less inertia than if the whole blade is being pitched. One disadvantage of Gurney flaps is however the increase in aerodynamic noise from the free ends of the Gurney flaps and from the gaps in the blade where the Gurney flap is positioned.
Within the aviation industry leading-edge droop and trailing-edge flap have been investigated and used. Examples of airfoil characteristics obtained by such devices are shown in [5] being exemplified in FIG. 2 and FIG. 3. It is the variation of the same effects that are desired with this new invention. Also, the aerodynamics of micro-air-vehicles have been investigated, where flexible airfoils have been proposed, [4].